[server] Add a benchmark for a simple handler, refactor tests.
[server] Add a benchmark for a simple handler, refactor tests.

It is good to keep an eye on overhead. With current implementation it is
relatively small (~8 microsec per request on my laptop). Absolute values doesn't
really matter though. What will matter is how it changes when more feature are
added.

R=tandrii@chromium.org, ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org
BUG=959427

Change-Id: I6a2b89befe36a15220b096e08a3f16bc220d0112
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1618587
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go